Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

mm: resolve faulty mmap_region() error path behaviour

The mmap_region() function is somewhat terrifying, with spaghetti-like control flow and numerous means by which issues can arise and incomplete state, memory leaks and other unpleasantness can occur.

A large amount of the complexity arises from trying to handle errors late in the process of mapping a VMA, which forms the basis of recently observed issues with resource leaks and observable inconsistent state.

Taking advantage of previous patches in this series we move a number of checks earlier in the code, simplifying things by moving the core of the logic into a static internal function __mmap_region().

Doing this allows us to perform a number of checks up front before we do any real work, and allows us to unwind the writable unmap check unconditionally as required and to perform a CONFIG_DEBUG_VM_MAPLE_TREE validation unconditionally also.

We move a number of things here:

1. We preallocate memory for the iterator before we call the file-backed memory hook, allowing us to exit early and avoid having to perform complicated and error-prone close/free logic. We carefully free iterator state on both success and error paths.

2. The enclosing mmap_region() function handles the mapping_map_writable() logic early. Previously the logic had the mapping_map_writable() at the point of mapping a newly allocated file-backed VMA, and a matching…